1. Field of the Invention
The present invention generally relates to fiber optic sensors for remote telemetry and, more particularly, to self-compensated fiber optic sensors, especially for the purposes of measuring fluid flow and shear stresses resulting from fluid flow.
2. Description of the Prior Art
Fiber optic sensors have been known for a number of years and are much preferred for making remote measurements of temperature, pressure and other conditions such as strain, flow rate, shear forces and the like, particularly in harsh environments. Performing telemetry using radiant energy carried by fiber optic light guides is inherently free from electromagnetic noise and interference and has proven highly reliable. Moreover, very small, inexpensive and highly robust sensors have been developed which are easily calibrated and provide extremely high sensitivity and accuracy through use of interferometric techniques.
Many designs for fiber optic interferometric sensors are known and many variations and implementations have been developed. However, the basic arrangement of the most successful of these designs generally involve the formation of a reflective surface near the end of a fiber optic cable which is used to both supply light to the sensor from a remote location and return light to the remote location after it is passed through the sensor. The basic principle of operation of such sensors is that the end of the fiber optic cable provides a partially reflecting surface allowing some light to pass to and be reflected by a further reflective surface spaced a very short distance from the end of the fiber optic cable thus forming a gap between reflecting surfaces. The sensor is configured in such a way that the length of the gap is variable with the parameter of interest. Thus the light reflected from the respective surfaces will have two components; one delayed with respect to the other and which will form an interference pattern in which regions of reinforcement or cancellation will be observable and which will vary strongly with potentially minute changes in the gap length. Other arrangements using other phenomena such as wavelength separation are also known.
To provide for the gap length to be reliably established while allowing variation thereof with any of a plurality of parameters of interest, the sensor structure of choice generally and most basically comprises a tube with optical fibers inserted into opposite ends thereof to be aligned in close proximity while forming a gap and the tube bonded to the respective optical fibers to fix the relative positions of the optical fibers. However, the important physical feature of a fiber optic sensor of this type is the gap between reflective surfaces and the tube housing, while generally convenient, is not necessary to the basic principles of a fiber optic sensor.
An important and frequently desirable measurement for which design of sensors of any type is difficult is that of sensing flow rate or shear stresses caused by fluid flow over a surface. For example, a significant fraction of the total resistance to motion of airplanes and ships is due to surface or skin friction while the availability of skin friction transducers is limited. Further, incidental effects of temperature and pressure are also generally unavoidable (e.g. due to Bernoulli and frictional heating effects) when measuring fluid flow across a surface. It is also very difficult to apply a sensor of any type to such a measurement, especially if direct measurement of skin friction or flow rate is to be made consistent with minimal interference with the measured parameter. Whether measurement of fluid flow rate/velocity or skin friction/shear forces are made directly or indirectly, the sensor must necessarily intrude upon the interface of the surface and the fluid and can thus potentially disrupt the parameter being measured and may not be reliable except in particular flow regimes. For example, Stanton tubes, Preston tubes and surface hot wire techniques are not reliable for complex three-dimensional or otherwise irregular flows (e.g. due to irregular surfaces, injection or suction of fluids or impinging shocks and direct measurements usually involve a floating head replacing a portion of the surface over which the flows take place or extending into the flow which is difficult, if not impossible, to provide without introducing at least irregularities in the surface. In general, however, direct measurements are less intrusive upon the flow regime and are thus generally preferred.
Because of the possibility that measurement of shear force or flow rate may interfere with the fluid flow, designs for such sensors have been developed of both the nulling and non-nulling types. A nulling sensor allows for motion or deflection of the floating head of the sensor but restores the floating head to a given position; the measurement being a function of the restoration force. That is, if the floating head remains in a given position, the interference with the measured parameter will at least be essentially constant or consistent since the shape of the surface will be unchanged at any flow regime. However, such nulling measurement arrangements are complicated and expensive while compromising reliability and response time. Non-nulling arrangements are far more simple, reliable and economical but, as pointed out above, may compromise the flow regime in unpredictable ways. Both nulling and non-nulling types of sensors, regardless of the measurement hardware are often subject to errors caused by temperature and or pressure variations which are unavoidable, as also noted above, particularly if of the fiber optic type.
However, a floating head, non-nulling fiber optic sensor for flow rate and shear forces is known an disclosed in the above-incorporated U.S. Pat. 6,246,796. This sensor uses a cantilevered arm to support a tethered floating head and uses fiber optic cables to conduct light to be reflected from surfaces of the floating head or parts of the support therefor. To provide temperature compensation, two or more optical fibers are symmetrically placed on opposing sides of the cantilever such that variations in geometry due to temperature will occur at both measurement gaps and can be approximately compensated by processing (e.g. subtracting) to remove the common mode component from both measurements. However, it is recognized in that disclosure that the floating head, even if tethered, may remain more subject to displacement from pressure than from shear forces and the sensor relies upon relatively greater stiffness along the length of the cantilever than in the direction of motion of the floating head to counteract effects of pressure. This patent also acknowledges extrinsic Fabry-Perot interferometer (EFPI) sensors using a construction including a glass tube for respectively locating the ends of partially reflecting fiber optic elements as discussed above but also notes that slight pressure sensitivity remains.